The present invention relates to a compression molding process for forming orthopaedic implant prosthesis bearings having increased wear resistance and improved mechanical properties. The present invention particularly relates to improved bearings for use in orthopaedic implant prosthesis and to methods for making polyethylene bearings by molding a cross-linked preform via the application of sufficient heat and pressure in such a way as to induce orientation in molded bearings.
Ultrahigh molecular weight polyethylene (UHMWPE) has been the material of choice for articulating surface applications for three decades. Such UHMWPE resin is commonly used for implantable prosthesis bearings, such as acetabular bearings, glenoid bearings, tibial bearings, and the like, for use in hip, knee, shoulder and elbow prostheses. In that time many improvements have been introduced, most notably irradiation of the polyethylene to induce cross-linking. In fact, the improved wear characteristics of the polyethylene have been largely attributed to such cross-linking procedures. Typically, a bar stock or preform, or a molded or machined bearing is irradiated and subsequently heat treated or heat annealed. The irradiation generates molecular cross-links and free radicals. Such cross-linking creates a 3-dimensional network in the polymer which renders it more resistant to abrasive wear in multiple directions. In addition, the free radicals formed upon irradiation of UHMWPE can also participate in oxidation reactions, which reduce the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement, and an increase in wear rate. The free radicals may be very long-lived, often several years, so that oxidation can continue over an extended period of time. Processes that tend to substantially eliminate residual free radicals induced by such irradiation tend to provide polyethylene with improved oxidation resistance. Typical processes for quenching free radicals in UHMWPE induced by irradiation involve elimination of the free radicals with heat treatments.
The bearings may be formed from polyethylene by direct compression molding processes or by machining the required bearing shapes from mill shapes, such as sheet or bar stock. Molding processes may be performed on unirradiated or irradiated polyethylene.
Processes to improve the material properties of the polyethylene, such as toughness, and the like, are yet sought. It is appreciated that such processes desirably do not compromise the advances in wear properties that have been made. Wear-resistant materials reduce wear debris-associated maladies, such as bone and soft tissue deterioration, wear debris-induced osteolysis, and the like. Such maladies may lead to implant loosening and possibly necessitate revision surgery.
Reference is made to a number of prior art references as follows:
1. U.S. Pat. No. 5,728,748, and its counterparts all relating to the same application, xe2x80x9cNon-Oxidizing Polymeric Medical Implant,xe2x80x9d to Sun, et al.
2. U.S. Pat. No. 5,879,400, xe2x80x9cMelt-Irradiated Ultra High Molecular Weight Polyethylene Prosthetic Devices,xe2x80x9d to Merrill et al.
3. U.S. Pat. No. 6,017,975, xe2x80x9cProcess for Medical Implant of Cross-Linked Ultrahigh Molecular Weight Polyethylene Having Improved Balance of Wear Properties and Oxidation Resistance,xe2x80x9d to Saum, et al.
4. U.S. Pat. No. 6,228,900, xe2x80x9cCrosslinking of Polyethylene for Low Wear Using Radiation and Thermal Treatments,xe2x80x9d to Shen et al.
5. U.S. Pat. No. 6,168,626, xe2x80x9cUltra High Molecular Weight Polyethylene Molded Article for Artificial Joints and Method of Preparing the Same,xe2x80x9d to Hyon et al.
6. U.S. Pat. No. 6,245,276, xe2x80x9cMethod for Molding a Cross-Linked Preform,xe2x80x9d to McNulty et al.
7. U.S. Pat. No. 6,281,264, xe2x80x9cChemically Crosslinked Ultrahigh Molecular Weight Polyethylene for Artificial Human Joints,xe2x80x9d to Salovey et al.
The above references teach the general concepts involved in forming or consolidating polyethylene resin directly into a component or a stock form from which the component is made, gamma or other irradiation of the component or the stock form, and subsequent heat treating (including annealing or remelting) of the component or stock form. The above references also teach the general concepts of compression molding and the appropriate apparatuses used therein. The disclosures of these above-listed references are incorporated herein for purposes of establishing the nature of polyethylene resin, the irradiation steps and options, and the heat treating steps and options.
The present invention provides UHMWPE bearings with improved mechanical properties, improved oxidation resistance and increased wear resistance. The polyethylenes prepared by the processes of the present invention also reduces the amount of wear debris generated from UHMWPE bearings. Typically, the polyethylene may be ultrahigh molecular weight polyethylene (UHMWPE), although it will be appreciated that the processes of the present invention may be used with various types of polyethylene.
The present invention is directed to a process for molding a bearing, near net-shape bearing, or net-shape bearing from a preform, which has been previously cross-linked, by obtaining such a preform and placing it in a press mold defining the desired bearing shape, or near net-shape, and applying heat and pressure in the mold to form the bearing in such a way as to orient the polyethylene. What is meant herein by the term xe2x80x9cnet-shape bearingxe2x80x9d is a bearing that is in a shape or condition that is satisfactory for use in a prosthetic implant upon removal of the bearing from a compression molding die without requiring any additional machining. The term xe2x80x9cnear net-shapexe2x80x9d, on the other hand, is meant herein to define a bearing which requires a small degree of further manipulation, such as machining, to produce the final bearing with such further manipulation being performed on surfaces of the bearing other than the articulating (i.e., bearing) surface of the bearing. What is meant herein by the term xe2x80x9cbearingxe2x80x9d is an orthopaedic implant prosthetic bearing of any type, condition, shape, or configuration. As such, the term xe2x80x9cbearingxe2x80x9d, amongst others, includes both net-shape bearings and near net-shape bearings.
The present invention is also directed to a compression molding process that induces biaxial orientation in the molded bearing to improve its wear characteristics and mechanical properties. In some embodiments, the preforms may be made from consolidated UHMWPE stock which has been irradiated. In other embodiments, the UHMWPE stock may be pre-annealed or pressure crystallized, or a combination thereof, to further enhance its mechanical properties. In an alternate embodiment, the irradiated UHMWPE preform may be heated or otherwise treated to substantially quench free radicals present in the irradiated preform.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of invention exemplifying the best mode of carrying out the invention as presently perceived.